1. Field of the Disclosure
The present disclosure relates generally to electrophotographic imaging devices such as a printer or multifunction device having printing capability, and in particular to a mechanism for counterbalancing forces that are presented onto the doctor blade of an electrophotographic device's developer unit by end seals thereof.
2. Description of the Related Art
Laser printers utilize a light beam which is focused to expose a discreet portion of a photoconductive or image transfer drum in an attempt to attract printing toner to these discreet portions. This photoconductive drum assembly is made out of highly photoconductive material that is discharged by light photons typically embodied by a laser. Initially, the drum is given a charge by a charge roller. As the photoconductive drum revolves, the printer shines a laser beam across the surface to discharge certain points. In this way, the laser “draws” the letters and images to be printed as a pattern of electrical charges, forming an electrostatic latent image. The system can also work with either a more positively charged electrostatic latent image on more negatively charged background or a more negatively charged electrostatic latent image on a more positively charge background.
When the toner becomes electrostatically charged, the toner is attracted to exposed portions of the photoconductive drum. After the data image pattern is set, charged toner is supplied to the photoconductive drum. Because of the charge differential, the toner is attracted to and clings to the discharged areas of the drum, but not to the similarly charged “background” portions of the photoconductive drum. With the image data toner pattern on the photoconductive drum, the drum engages a sheet of paper or media moving adjacent thereto. The paper or other media may be driven by a transport belt or transfer roller, which is oppositely charged to the toner causing it to transfer to the paper or other media. This charge is stronger than the charge of the electrostatic image, so the transfer roller pulls the toner away from the surface of the photoconductive drum. When the printing media passes beneath the rotating photoconductive drum, the toned image is transferred to the media. The transferred toner is subsequently fused to the paper typically by application of heat and pressure.
Toner to be used is initially stored in a removable container often located in a toner cartridge. The printer gathers the toner from the toner container and supplies it to a developer unit using paddles and transfer rollers. The developer unit may be located in the toner cartridge or separate therefrom. A developer roll is a charged rotating roller, typically with a conductive metal shaft and a polymeric conductive coating, which receives toner from a toner adder roll positioned adjacent the developer roll in the developer unit. Due to electrical charge and mechanical scrubbing, the developer roll collects toner particles from the toner adder roll. A doctor blade assembly engages the developer roll to provide a consistent coating of toner along the length and surface of developer roll, by scraping or “doctoring” excess toner from the developer roll and metering a thin layer of toner on the developer roll surface. In turn, this provides a consistent supply of toner to the photoconductive drum. Without a doctor blade, the coating of toner on the developer roll may be inconsistent, too thick, too thin or bare, thereby causing the amount of toner presented to the latent image on the photoconductive drum to be inconsistent and the level of darkness of the printed image may vary as a result, which is considered a print defect.
One challenge with existing doctor blade assemblies is that of providing a consistent seal generally around the location where the doctor blade assembly, the developer roll and the developer unit housing meet. Seals, including J-seals, have been found to effectively inhibit toner leakage in this area. However, J-seals prevent toner leakage through contact with the doctor blade and developer roll. Such contact results in nonuniform nip pressure between the doctor blade and the developer roll by providing a greater force at the ends thereof than the middle. This greater force at the ends creates a thinner layer of toner and higher charged toner on the toner layer disposed on the developer roll which increases background printing along the ends of the printed image. The greater force between the doctor blade and the developer roll also generates more heat at the ends of the doctor blade, thereby resulting in shorter life of the doctor blade and developer unit. The greater force at the ends of doctor blade increases the wear rate of the doctor blade and shortens the life of the doctor blade. The heat generated may potentially cause toner to melt on the developer roll, which will damage the developer unit.
Based upon the foregoing, there is a need for a developer unit for an electrophotographic imaging device having improved printing performance and useful life.